EDIT: Some more information from Catija: This screen element is not currently customizable, so if we wanted it customized, SE would have to implement a new feature. Here Catija indicates that the current SE thinking is that this particular screen element ought to be something which looks the same across the whole SE network. But here she indicates that on ...

As a data point for our Stack Exchange overlords, I have twice now tried to migrate a question from math.SE to math.SE because I didn't realize it was already there. I'd really like to get a striking difference in color and background.

I suggest that we leave the job to Stack Exchange's design team. I'm not sure why we are supposed to come up with a solution ourselves; we are not professionals, and surely SE has someone on staff whose job is coming up with good site designs. I like many of their designs that were on other sites in the network before this change.
What worries me, though, ...

According to the post on Meta Stack Exchange1 (and the stuff I was told in chat) it seems that color of tags can be changed. (To be more specific, Catija mentioned as an example of a site which changed looks of tags from defaults Mi Yodeya.)
Maybe simply changing background colors for the tags would help to made the site distinguishable from others. Perhaps ...

We could consider some kind of background theme. I think MSE's graph paper looks pretty good. Of course, the goal would be to come up with something good which contrasts to the MSE graph paper.
If we did this, I imagine we'd want to solicit particular suggestions from the community and then vote on them.

We could consider choosing some kind of art in the header of the page.
MO has always had a "plain" sort of theme, but maybe it's time for something new.
If we did this, I imagine we'd want to solicit particular suggestions from the community and then vote on it.